1. Field of the Invention
The present invention relates to an electronic musical instrument that employs waveform interpolation to produce naturally sounding synthesized musical tones.
2. Description of the Related Art
At present there are various types of electronic musical instruments whose performance duplicates, as nearly as is possible, or even exceeds that of natural musical instruments.
Such electronic musical instruments generally have tone generation systems for the production of musical tones that correspond to notes.
To provide the profusion of expressive sounds that are available with natural musical instruments, most of the musical tones that these electronic musical instruments produce can be given a variety of timbres by superimposing harmonics, as necessary, on the basic waveforms that correspond to individual notes.
A tone generation system that is employed for this purpose stores waveforms in a read only memory (hereafter referred to as a "ROM"). Following the selection of a note by the manipulation of switches or keys at a panel or a keyboard, the waveform that corresponds to the selected note is read from an address that is specified by a central processing unit (hereafter referred to as a "CPU"). The waveform is then amplified, and after a D/A conversion is performed, the converted waveform is output as an acoustic signal.
When, in the above described manner, the waveforms stored in the waveform ROM are read sequentially, and multiple read-out waveforms whose timbres differ are output to produce a desired musical tone, the timbres are changed drastically and a discontinuous, unnatural sound is produced.
For example, the waveform for an attenuated tone tends first to provide a drastic timbre change at the beginning of tone production, and then to gradually change until it has become a stationary waveform. If waveform data that correspond to the stages of such timbre changes are stored in a ROM and are extracted sequentially, a naturally sounding musical tone cannot be provided.
In an effort to preclude the production of unnatural musical tones, electronic musical instruments have been constructed that perform a crossfade by altering timbres, at constant time intervals, so as to smoothly change from one timbre, i.e., waveform, to another.
A crossfade is a process that provides a smooth exchange of timbres by performing a fade-out effect, which involves the gradual weakening of a preceding timbre, and a fade-in effect, which involves the gradual strengthening of a succeeding timbre, while partially overlapping the two effects.
In certain systems that employ conventional techniques to change timbres at a constant time interval, however, the timing for waveform switching is fixed. Therefore, when the switching time interval is long, much of the waveform change data are lost if there is a drastic timbre change.
When the switching time interval is short, a system can cope with drastic timbre changes; but when a timbre change is performed gradually, the efficiency of waveform compression is degraded and the System does not function economically.
In those systems that change timbres within variable time periods, a storage means is required to hold the data that are employed to determine the timing for the waveform switching.
A ROM is generally employed as such a storage means; however, accessing the ROM and reading the necessary data require much time.